About three-quarters of all U.S. employees in the private sector get paid time off, according the Bureau of Labor Statistics. But the number of days pales in comparison to what those in other developed countries get. And yet, American workers still don't take advantage of their paid time off.

A new study developed by specialized staffing firm OfficeTeam found that one-third of U.S. workers don't use all their vacation days.

An independent research firm who conducted the study on behalf of OfficeTeam, which is a Robert Half company, interviewed 436 adults working in an office environment and uncovered the top reasons why two in five people do not use all their paid time off.

About 38% said they were "saving days in case they need them later," while 30% said they worry about falling behind at work. The remaining respondents said, "I don't like taking time off" (12%), "I don't get any vacation time" (10%), "My manager would frown upon it" (3%), and "Other" (7%) as their primary reason for not taking all of their allotted vacation time.

"Managers may be hesitant to take extended time off because they fear their absence will be a hardship on staff and other executives," says Robert Hosking, executive director of OfficeTeam. "Other workers may see their managers or colleagues skipping vacations and feel guilty about taking time off themselves. They don't want to look like they're not a team player or a slacker."

Still, others may have more personal reasons for not taking enough time off, Hosking explains. For example, financial reasons may prohibit a costly trip or extended time off.

"With many workers taking on increased workloads, it isn't too surprising that a lot of people are leaving vacation time on the table," Hosking says. "They may just be too busy to even think about taking vacation. But having more on your plate makes it even more important to take time off for work-life balance."

Interestingly, 34% of surveyed respondents said the biggest mistake they made on their latest vacation was that they didn't take enough time off.

"Time away from work allows workers to recharge and regain motivation for their jobs. By taking a vacation, you can return to work with a fresh perspective and renewed energy, which can boost your productivity and effectiveness," he says.

The good news is that more and more employees say they are not checking in with work when they do go away.

In 2005, only 21% of workers said they did not check in with the office while on vacation. By 2012, that number rose to 51%.

"It may be that people are finally realizing that ideally you should completely disconnect from work while on vacation to truly enjoy the time off," Hosking says. "Workers may also be better at planning and preparing for vacations, so they don't need to check in while away." That may include tying up loose ends on projects in advance and providing trusted colleagues with all the resources necessary to take over key projects.

"Expectations about whether or not you need to check in with work while on vacation also come from the top down," he adds. "Managers may be setting the example of how you can totally disconnect from the office during your time off. In addition, many bosses realize that in order to keep their best employees motivated and avoid them from burning out, they need to give them time to take real breaks."

However, some people still feel pressure to check in while on vacation because they're afraid of being "out of sight, out of mind." But, he says, spending vacation hours attending to business can be counterproductive. "The benefits of a vacation are lost when you're constantly thinking about and attending to office matters."

To avoid feeling as if you need to check in, Hosking suggests preparing for a vacation as if you won't be available, rather than planning to check in. "You should establish expectations about whether or not you're planning to check in, letting others know, and following through with your plans," he says. "And if you must check in, establish 'office hours' so colleagues are aware of the days and times you're available instead of being on call the entire time."